1. Field of the Invention
Aspects of the present invention relate to a supported catalyst, and more particularly, to a supported catalyst having excellent membrane efficiency in electrodes for fuel cells due to a uniform alloy composition of catalyst particles and supported catalysts which are not agglomerated, a method of preparing the supported catalyst, a cathode electrode including the supported catalyst, and a fuel cell including the cathode electrode.
2. Description of the Related Art
As used generally in the art, the term “supported catalyst” refers to a catalyst composed of a catalyst component and a porous catalyst support to which the catalyst component adheres. The porous catalyst support typically has many pores, and thus has a very large surface area. Such a large surface area provides a large area in which many catalyst components can be dispersed. Supported catalysts are widely used to accelerate various reactions in various fields.
An example of a supported catalyst is a carbon supported metal catalyst. The carbon supported metal catalyst includes porous carbon particles as a catalyst support and catalytic metal particles as a catalyst component. Carbon supported metal catalysts are also widely used to accelerate various reactions in various fields.
An example of a carbon supported metal catalyst is a catalyst contained in an electrode for a fuel cell. More particularly, the cathode and/or anode of fuel cells such as a phosphoric acid fuel cell (PAFC), a polymer electrolyte membrane fuel cell (PEMFC) or a direct methanol fuel cell (DMFC) contain a carbon supported metal catalyst that accelerates an electrochemical oxidation of a fuel and/or an electrochemical reduction of oxygen. In such fuel cells, carbon particles act both as a catalyst support and as an electron conductor. Pt and Pt/Ru alloy, etc. are generally used as the catalytic metal particles.
In a conventional method of preparing a supported catalyst disclosed in U.S. Pat. No. 5,068,161, a catalyst metal precursor is adsorbed onto a catalyst support by adding a catalyst metal precursor solution into a catalyst support dispersion solution. The catalyst metal precursor which is adsorbed on the surface of the catalyst support is reduced to catalytic metal particles by adding a reduction solution to the mixture, and then the resultant is freeze-dried to prepare a supported catalyst powder.
As is well-known in the art, one of the factors affecting the activity of a supported catalyst is the total surface area of the catalytic metal particles supported on the catalyst support. The main factors affecting the total surface area of catalytic metal particles in a supported catalyst are the average size of catalytic metal particles and the amount by weight of catalytic metal particles. That is, for a given amount of catalytic metal particles over an entire supported catalyst, the total surface area of catalytic metal particles supported on the supported catalyst generally increases as the average size of catalytic metal particles decreases. In addition, for a given size of catalytic metal particles, the total surface area of catalytic metal particles supported on the supported catalyst generally increases as the amount of catalytic metal particles in a supported catalyst increases.
Thus, to increase the total surface area of catalytic metal particles in a supported catalyst, the catalytic metal particles may be made smaller and/or the amount of the catalytic metal particles supported on the catalyst support may be increased.
For example, as the activity of a carbon supported metal catalyst contained in an electrode of a fuel cell such as PAFC, PEMFC, and DMFC increases, the power density of energy generation systems in the fuel cell increases while energy conversion efficiency is appropriately maintained. Accordingly, the ratio of the amount of generated power to costs for manufacturing a fuel cell stack increases, and the ratio of the amount of generated power to the weight or volume of the fuel cell stack increases.
However, in supported catalysts prepared according to conventional methods, as the loading amount of catalytic metal particles increases, the average size of catalytic metal particles supported also generally increases. For this reason, the ability to improve the catalytic activity of a supported catalyst by controlling the average size of catalytic metal particles and the loading amount of catalytic metal particles is limited. Moreover, in supported catalysts prepared according to conventional methods, it is difficult to reduce the average size of catalytic metal particles even by reducing the loading amount of catalytic metal particles. Thus, a technique for reducing the average size of catalytic metal particles supported on a catalyst support in a loading amount of catalytic metal particles more than or equal to the conventional loading amount of catalytic metal particles would be advantageous.